b) $...=\frac{\sqrt{a}-\sqrt{a}+1}{\sqrt{a}(\sqrt{a}-1)}:\frac{(\sqrt{a}-1)(\sqrt{a}+1-(\sqrt{a}+2)(\sqrt{a}-2)}{(\sqrt{a}-1)(\sqrt{a}-2)}$
$=\frac{1}{\sqrt{a}(\sqrt{a}-1)}:\frac{(a-1)-(a-4)}{(\sqrt{a}-1)(\sqrt{a}-2)}$
$=\frac{1}{\sqrt{a}(\sqrt{a}-1)}.\frac{(\sqrt{a}-1)(\sqrt{a}-2)}{3}$
$=\frac{\sqrt{a}-2}{3\sqrt{a}}$